[SM-PLUGINS] Update for captcha plugin - new google recaptcha API

[tlhackque <tlh...@ya...>]

I've added a backend for the CAPTCHA plugin that supports the new Google
reCAPTCHA API (checkbox captcha, etc). It's packaged as a .tar file that
can be dropped onto the sqirrelmail directory.

Where do I post the .tar file for the community?  The patches URL on
squirrelmail.org points to a stale page on sourceforge.

I guess someone will ask: it's a new backend because the parameters are
different, the api requires https, and some people may not want the new
version. The new backend is checkbox_recaptcha.  Still, it's an easy
upgrade - edit the checkbox_recaptcha config file and switch the backend
name.  Things are just different enough to require a different config,
but not so different as to be hard to make the change.

Caveat: I provided code to detect and use the 1.5.2+ template system,
but as I don't run it, it's not tested.  The normal code works for me.

The kit also includes an optional patch allowing IPv6 wildcards in the
hide/show by IP address lists.  I didn't add IPv6 CIDR support as (a) I
don't need it and (b) it's a bit of a pain to deal with 128 bit addresses.

Finally, if you haven't hard about the new reCAPTCHA, see
http://googleonlinesecurity.blogspot.com/2014/12/are-you-robot-introducing-no-captcha.html
In short, it's a better user experience - most of the time.

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